Damage and destruction of valuable field crops and horticultural cultivations due to infestation by plant parasitic nematodes is a serious problem to agriculture, causing considerable economic loss. In the past, control of nematodes has generally required the field application of chemical nematicides. However, chemical nematicides invariably lose their effectiveness due to nematodes developing resistance as well as build-up of soil organisms that can cause their degradation. In addition, application of chemical nematicides is often environmentally undesirable and, in recent years many have been withdrawn from the market due to their toxicity and persistence in the environment.
Biological control of nematodes would be an attractive alternative to the use of chemical nematicides and many laboratory experiments have indicated that there is good potential for the use of nematophagous fungi in such an approach. However, when experiments have been extended to non-sterile soil in a genuine cropping cycle, invariably the results have been disappointing. No doubt, as a consequence, Stirling (1988, "Biological Control of Plant Parasitic Nematodes" in "Diseases of Nematodes" Vol. II [Eds G. O. Poinar and H-B Jansson] pp 93-139, CRC Press Inc., Boca Raton, Fla.) has recently predicted that "biological control will remain an insignificant component of nematode control programmes, at least until the turn of the century". The reason usually advanced for such experimental failure is that the soil ecosystem involves a highly complex interaction between different microorganisms, and that it would be very difficult to manipulate such a system with economically feasible additions of other microorganisms (Van Gundy, 1985, In "Biological Control in Agricultural AIPM Systems"; (Eds M. A. Hoy and D. C. Herzog) pp 468-478, Academic Press.
Over 150 species of soil fungus are known to attack and destroy nematodes and extensive reviews are available in the literature (Jatala 1986, Ann. Rev. Phytopathol. 24, 459-489). Nematophagous fungi are broadly divided into two groups, (1) nematode-trapping fungi which produce mechanical or adhesive traps; and (2) endoparasitic fungi which infect nematodes when their conidia (spores) are ingested or adhere to the cuticle.
In the past there have been attempts to use nematode-trapping fungi, which are dependent on the presence of soil nematodes for growth, as biological control agents. An example of such is the commercial preparations based on Artrobotrys spp. However, these preparations have met with very limited success.
Some saprophytic soil fungi can parasitism nematode eggs (Stirling, supra). Because fungi of this type can have a dramatic effect on soil nematode populations, they are among the most interesting for their biological control (Sterling & Manakau, 1978, Mycologia 70 774-783). Jatala et al. (1979 J. Nematol. 11, 303) claimed that Paecilomyces lilacinus was an effective parasite of the eggs of Meloidogyne spp. and Globodera spp. A methodology was developed for the application of a Peruvian isolate of P. lilaninus and a large number of investigators in different countries were involved in trials with this particular organism. Whilst some results are apparently encouraging, little has been published of this work and that which has been, in many cases, is inconclusive (Jatala, 1986 Ann. Rev. Phytopathol. 24, 459-489).
A review of the literature over the last five years suggests that while there has been substantial laboratory and glasshouse experimentation on biological control of plant parasitic nematodes, the overall picture is confused and lacks consistent demonstration of any immediate practical application to agriculture. In regard to P. lilacinus spores obtained from mycelia grown on rice hulls and rice bran mixture, or chopped water lily and incorporated into the soil gave significant control of Meloidogyne incognita infesting okra (Davide & Zorilla, 1985, Phil. Agric. 68, 493-500). Good results were also found against M. incognita on corn, tomato and okra in glasshouse experiments in comparison with the chemical nematicide Aldicarb (Ibrahim at al., 1987, Nematol. Medit. 15, 265-268). However, inconclusive results against M. incognita were obtained when mixtures of P. lilacinus and Pasteuria penetrans were used in experiments involving Vicia villosa (Dube & Smart, 1987, J. Nematol. 19, 222-227). Hewlett et al. (J. Nematol. 20, 578-584, 1988) found that P. lilacinus did not control Meloidogyne javanica over a two year field experiment on tobacco and actually produced lower yields than all other treatments. In another group of experiments, Culbreath et al. (Nematotrop. 16, 153-166, 1986) tested the effects of a mixture of P. lilacinus and chitin for controlling Meloidogyne arenaria. The levels of material added, however, would prohibit any commercial application. Brazilian experiments by Novaretti et al. (Nematol. Brasil. 10, 133-144, 1986) led to substantial decreases in yield and no control of Pratylenchus spp. on sugar cane. Such inconsistent results in experiments using P. lilacinus indicate underlying problems in present approaches to its wide spread application as a biological control agent for nematodes. It has been suggested (Cabamilis et al., 1989, J. Nematol. 21, 164-172) that such variability in results may be caused by mutations. They go on to attribute loss of effectiveness as nematophagous agents in Peruvian isolates to culture material being kept with periodic transfer on media in the absence of nematodes.
Even in regard to some apparently successful field experiments using Paecilomyces lilacinus, Stirling (supra) suggests "that there is lack of convincing evidence that the introduced fungus was responsible for the observed reductions in nematode population".
To summarise existing technical knowledge, it is well established that many naturally occurring soil fungi are capable of parasitising and killing nematodes. However, use of these for biological control in realistic agricultural contexts in the past has met with mixed success and in a number of cases the form of the fungus administered to the soil is such that it would not be applicable to commercial use.